When a fuse blows, an arc is developed which, if it spreads to the metal surfaces of the fuse terminals, will vaporize the surface layer thereof and create fuse exploding pressures. In an AC circuit, the arc generally becomes extinguished as the AC current drops to zero and may not restrike or cause rupture of the fuse if the pressures and temperatures in the fuse cavity can be held within acceptable limits. As fuse structures are made progressively smaller, it becomes more difficult to keep these parameters within desired limits.
There is a need in the printed circuit art for fuses of substantial voltage rating, i.e. 125-250 volts, and characterized by as small an overall dimension as possible. Such requirements are inherently in conflict, since a blowing fuse tends to generate rupture forces as a result of gas evolution and heating during the traveling of the arc along the fuse wire path and hence fuses capable of withstanding substantial restrike voltages during blowout typically must be fashioned with length greater than otherwise desired to allow the arc to extinguish and prevent rupture of the fuse casing. Should the casing rupture, there is an attendant fire hazard, as well as an attendant danger of damage to components on the printed circuit board itself. Printed circuit fuses should also have adequate protection against the entry of spray or dip solvents commonly used in the cleaning of printed circuit boards after final assembly of the components thereon.
To the applicant's knowledge, prior to the present invention there has not been designed a reliable sealed fuse much smaller than previous designs and capable of withstanding high energy fuse blowing conditions without destruction of the fuse housing. For example, there is a need for a reliable miniature printed circuit fuse which for a steady blowout current of 50 amps and 250 volts or equivalent energies can be made reliably as small as about 0.4 inches or less in overall length and even less in height and with a terminal spacing of the same dimension if desired (as when the terminals project axially from the fuse body ends and then bend downwardly). There has heretofore been developed cylindrical fuses with depending terminals within the boundries of the fuse and having a diameter of about 0.3 to 0.4 inches. The width of the fuses thus had to be greater than the terminal spacing and the height of the fuse was equal or greater than its width. Thus, at present, printed circuit fuses capable of withstanding such energies are relatively large, bulky fuses with cylindrical insulating bodies. Also such cylindrical fuses are too bulky for mounting on carrier strips wound on dispensing reels which can be conveniently inserted into automated machinery which automatically insert the fuses into the printed circuit board.
Fuses used on printed circuit boards generally comprise an insulating body defining a cavity or compartment in which a fuse element is suspended between fuse terminals which often project from opposite axial ends of the body and terminate in parallel confronting terminal ends pluggable into socket openings in the printed circuit board. Since the general objective in printed circuitry is miniaturization, it is desirable that the fuse itself occupy as little space on the printed circuit board as possible.
It is frequently required of some low amperage fuses that they use fuse wire of very small diameter, such as the order of 0.0003 inches, for example. There is an inherent difficulty in fabricating fuses using such delicate fuse wires since the tensioning and positioning of such elements during delicate soldering operations is typically a manual operation resulting in substantial labor costs. Thus, an adequately miniaturized high voltage fuse of relatively low blowout current which could be manufactured inexpensively by automated methods would be a useful contribution to the art. The present invention involves a unique design for a sealed fuse permitting a reliable fuse with such small fuse wire sizes and of a given current and voltage rating to be made even by automated means much smaller than conventional fuse designs of the same rating.
While some prior art miniature fuses have some features in common with the present invention, such as insulating bodies with cavities and fuse element lay-in grooves, axially projecting terminals and enclosing sleeves, these features are found separately in different fuses and have not been combined in the manner of the various features of the present invention. Also, the sizes and relationships of the grooves, cavities and terminals used in the present application are quite different from those of prior art fuses.
U.S. Pat. No. 3,913,051 issued to Manker et al discloses a miniature fuse comprising a body of insulating material having a small depression or well formed therein and having a fuse element which spans the well and rests upon metallized support surfaces on the body beyond the well. A pair of terminals have inner ends which overly and are secured by solder joints to the end portions of the fuse element. Shrink tubing tightly envelopes this entire assembly to seal the fuse interior from the ambient conditions of the fuse. Transparent tubing is used in the Manker et al fuse to allow visual detection of the blown fuses, but since the background for the fuse is the wall of the well behind the fuse, there is no clear view of the fuse element through the window produced by the transparent tubing.
The well in the Manker et al patent provides a space between the fuse element and the insulating body. This space is stated to be desirable to provide thermal isolation therebetween; however, in one form of the Manker et al invention, the tubing is shrunk into contact with the portion of the fuse element spanning the well. In such a case, the small well size provides a cavity for the fuse element which is under 10 percent of the overall volume occupied by the fuse. Another form of the invention is disclosed where the part of the shrink tubing overlying the central well-spanning portion of the fuse element is spaced from the central portion of the fuse element. The overall cavity size is still quite limited in this design and so it is unlikely that this fuse with a terminal spacing of 0.4 inches could withstand without rupture an arc in a 250 volt circuit. While some mention is made in the Manker et al patent of the fact that the shrink tubing is made of a flexible material, there is no mention or teaching in the patent that the tubing expands without rupture when the fuse blows so as to increase the size of the cavity to avoid the build-up of fuse rupturing forces as in the case of a fuse made in accordance with one of the features of the invention. If this were the intent of the flexible tubing, it is most likely the patent would have referred to such fact.
The prior art has used various techniques to increase the operating voltage of fuses by incorporating various arc quenching means therein. Thus, fuse elements have been surrounded by a suitable arc-quenching material. However, this approach is difficult to achieve in miniature fuses, or where very delicate fuse elements are used in the fuse. Another arc-quenching technique is to pass the portions of the fuse element immediately in advance of the points where they are soldered to the fuse terminals through restricted openings or grooves in the insulating material of the body involved, as shown by the fuse construction of U.S. Pat. No. 4,267,543, granted to Arikawa. This patent discloses a fuse structure employing a fuse element spanning a cavity defined between D-shaped insulating arc barrier-forming bosses in a cylindrical base portion of the fuse. The bosses are slotted to receive the fuse element and have recesses to receive and expose the terminals of the fuses to which the fuse element ends are soldered. A rigid cover overlies the base portion of the fuse. However, it is believed that the fuse design is inadequate to withstand without rupturing the pressures and temperatures present in a 250 volt circuit when made with a less than 0.4 inch exposed to arcing terminal separation. Furthermore, because the circuit plug in terminals are spaced parallel pins, the overall size of such a fuse would be much greater than the terminal spacing.
Applicant before the present invention designed and built fuse constructions constituting improvements on the Manker and Arikawa fuse designs. These fuse constructions comprise a housing including a base portion carrying the circuit plug-in terminals of the fuse and defining part of the fuse cavity and fuse element lay-in grooves at the opposite ends of the fuse cavity. A cover encloses the base portion of the fuse housing and supplies depending ribs which extend into the lay-in grooves so that the fuse element is surrounded on all four sides by masses of insulating material at each end of the fuse element immediately ahead of the point where the fuse element is soldered to the adjacent fuse terminal. The cover and base of the housing are ultrasonically welded together. This type of fuse construction, while believed to be an improvement over the Manker et al and Arikawa fuse designs, did not always withstand the fuse rupturing forces in 250 volt or other high energy circuits. Thus, prior to the present invention, there was still a need for a miniature fuse which could be readily manufactured, preferably by completely automatic assembly techniques, which was spray and or dip solvent resistant, could withstand high arc energies preferably as high as those present in 250 volt circuits without explosive rupture, and wherein a blown condition could be readily detected by visual inspection. However, the broader aspects of the invention are not limited to fuses for use in 250 volt circuits.